Many electronic devices such as message displays require isolation between their inputs and their internal circuits. Process control systems often require many moderate voltage inputs operating in the presence of noise and leakage currents. Economy, size and power consumption are important considerations when designing an interface circuit that is utilized many times in the electronic design.
Practical input circuits provide noise and leakage rejection through the use of low-valued resistors operating under high-power conditions. The resulting high-power operation results in a physically large and costly system that is prone to failure due to heat.